1. Field of the Invention
The present invention relates generally to the production of stepped shafts and more particularly, to a novel and improved method and apparatus for rolling an elongate solid metal member to rough-form a stepped shaft.
2. Prior Art
The term "stepped shaft" refers to an elongate shaft having at least two adjacent outer surface portions of different diameters separated by a substantially radially extending shoulder. The outer surface portions of differing diameters share a common longitudinally extending center axis. In essence the outer surface of the shaft has a "stepped" configuration.
Stepped shafts formed from solid metal members are utilized in many different applications. Stepped shafts of relatively large size, e.g. those having a major outer diameter of about 6 inches or more, are utilized as processing rolls in strip mills, as axles for large off-the-road vehicles, and for many other purposes.
While stepped shafts of relatively small size can be produced economically through conventional machining processes, the production of stepped shafts of relatively large size is presently quite expensive. Factors tending to increase the expense associated with the fabrication of large stepped shafts include the difficulties that are encountered in handling the heavy body of metal from which the shaft is to be formed and in transferring the metal body or workpiece from one work station to another as it is worked during different stages of formation. Still another factor resulting in high fabrication cost is the significant amount of machining time required to cut away unwanted metal from the body as reduced diameter regions are formed on the workpiece.
The only technique in commercial use today for forming large stepped shafts is that of machining. Machining is carried out with a workpiece supported in a lathe and with conventional cutting tools operating on the outer surface of the workpiece. Machining progresses relatively slowly and may require many cutting tool passes to form the desired shaft configuration.
Although machining is an acceptable production technique where a small amount of material is to be removed, machining is expensive and time consuming where it must be used for the entire operation of forming a forged member into a closely toleranced stepped shaft. Often as much as 25% of the rough-formed material must be removed during machining of a large stepped shaft, and this tedious work tends to consume a great deal of machine time and operator time. Where relatively large cuts of metal are being removed by machining, significant amounts of energy are consumed and considerable cutting tool wear occurs.
Other than machining, the only viable technique for producing large stepped shafts is that of rolling. Prior to the present invention, the only known rolling technique theoretically capable of producing large stepped shafts was that of cross-rolling. In cross-rolling, as that technique has been used to date, a pair of flat plates are spaced from each other a distance sufficient to accommodate an elongate, generally cylindrical workpiece therebetween. The flat plates each carry raised wedge-shaped die surfaces for engaging the outer surface of the workpiece. The plates engage opposite sides of the workpiece and are moved in unison in opposite directions to roll the workpiece as the die surfaces clampingly engage the outer surface of the workpiece. During the back and forth movements of the plates, they are gradually pressed more closely together to cause a corresponding progressive decrease in the diameter of the workpiece surface portion engaged by the plates.
Cross-rolling has been used to form relatively small stepped shafts and other small objects such as threaded bolt blanks. The most attractive advantage of cross-rolling is that the shaft or bolt blank is formed quite rapidly, often within about 10 revolutions of the workpiece and often during a single stroke of the relatively movable flat plates. Cross-rolling is also advantageous because of its high production speeds and because the workpiece can be maintained at a high temperature throughout the rolling process.
Unfortunately, cross-rolling has several drawbacks which are difficult to overcome and which have tended to limit the application of this technique to the production of small objects of relatively simple configuration. If it is necessary to produce shafts having several steps in diameter, the raised wedge die surfaces must be of complex configuration. One problem with complex die surfaces is that they tend to inhibit necessary axial displacement of the material being machined. Moreover, complex die surface configurations may cause some portions of the material being rolled to stretch far more than it should, or to be rolled more times than it should, thereby undesirably altering the properties of the material. Still another problem which obtains with the use of complex die surfaces and cross-rolling techniques is that the dies are of fixed configuration and cannot be used to form stepped shafts of widely differing configurations.
Another, problem encountered with cross-rolling is that of "center cracking." This term refers to the formation of a cavity near the longitudinal axis of the workpiece. The cavity is believed to be caused by the extreme compression which occurs between the opposed rolling plates and by a so-called tri-axial stress system which is set up in the workpiece. The problem is so severe that some shafts produced by cross-rolling are greatly diminished in strength. In order to successfully produce even small shafts by cross-rolling, it has been found necessary to exercise very close control over the entire rolling operation, including control over the design of tooling, the selection of workpiece temperatures, and the selection of force loads to be imposed on the workpiece.
While a number of other rolling techniques are known for use in the formation of hollow members such as baseball bats and tubular articles, these techniques are not suitable for use in the formation of large stepped shafts.